1. Field of the Invention
This invention relates to substrate chemical vapor deposition apparatus and, more particularly, to substrate loading, off loading and handling subsystems interacting with a processing subsystem.
2. Discussion of the Related Art
In the electronics art it has long been a practice to employ chemical vapor deposition techniques for depositing various materials on substrates or wafers as part of the process for manufacturing semiconductor devices. Chemical vapor deposition processes include passing of a reactant gas, which contains the material to be deposited, over the substrates for forming or growing a compound on the substrates through thermal reaction or decomposition of the various gaseous materials.
The equipment used to accomplish such a process may be of various configurations but will include the basic components of a reaction chamber, a heating system and a gas flow system. Batch processing equipment has been used to accomplish the chemical vapor deposition process in a production environment; it may be horizontal gas flow system or a vertical gas flow system. A horizontal gas flow system includes a susceptor located in a horizontally disposed reaction chamber with the reactant gas flowing in a horizontal path across the susceptor. In a vertical gas flow system, a horizontally disposed susceptor, or an upstanding multisurface barrel shaped susceptor, is located in a vertically disposed reaction chamber with the reactant gas flowing in a substantially vertical path past and around the susceptor. In either system, the susceptors are configured to support a multiplicity of relatively small substrates, i.e. in the neighborhood of 2 to 5 inches (5 to 12.5 cm) in diameter. While simultaneous deposition of materials on a multiplicity of substrates is desirable from a manufacturing standpoint, it has some drawbacks from a quality standpoint.
The first problem associated with batch processing relates to the reactant gas. As the gas flows over the surfaces of the substrates and the susceptor, deposition of the materials results in changes in the concentration of the materials in the reactant gas. Consequently, as the reactant gas flows across or over the length of these relatively large susceptors, across each individual substrate and across a multiplicity of such substrates, different rates of growth of the deposited layer of material may occur. A second problem is that of temperature control, which is critical at the elevated temperature needed for proper deposition. It is difficult, if not impossible, to control the temperature within the critical tolerances at all locations of interest within relatively large reaction chambers. This results in different deposition layer thicknesses from substrate to substrate and can result in a varying thickness on an individual substrate. Contamination is a third problem which can result from various factors, such as handling techniques in loading and unloading the substrates, introduction of the reactant gas into the reaction chamber, and the like.
These problems all contribute to significant difficulties as the uses of the semiconductor devices developed from the substrates become more sophisticated. To alleviate some problems, manufacturers are now using automated loading and off loading devices instead of manual techniques. In vertical gas flow systems, the upstanding barrel shaped structure is rotated about its vertical axis to rotate the multiplicity of substrates within the reaction chamber. Such rotation produces temperature and reactant gas flow averaging. However, there are practical limits which many believe will ultimately make the batch processing techniques unacceptable or at least undesirable. One of the limitations is that the equipment is not very well suited for handling larger diameter substrates. Increasing the size of the substrate causes some problems with regard to temperature differentials across the substrate, decreasing concentrations of the deposition material across the substrate, and the like. Single substrate chemical vapor deposition equipment becomes inherently more desirable than multisubstrate equipment as the manufacturers change to larger substrates, i.e. 6 to 8 inches (15 to 20 cm) in diameter or larger. One important consideration is the cost at risk when processing one substrate as opposed to simultaneous multisubstrate processing; that is, if something goes wrong, the monetary loss is far less with one substrate than it is with a plurality of substrates.
Various prior art components and subsystems have been used in single substrate processing chemical vapor deposition systems. For example, loading and unloading of substrates into such systems may be handled in various ways with the most pertinent prior art structure being a cassette elevator. The cassette elevator includes a vacuum chamber for receiving a plurality of substrates that are carried in a cassette supported on a platform. The platform is vertically movable by an elevating mechanism to bring the substrates one at a time into alignment with an access port. An isolation valve is located at the access port for closing the vacuum chamber except during extraction of the individual substrates. Both the elevating mechanism and the isolation valve provide a controllable environment for receiving and loading the substrates.
A vacuum transport station extracts the substrates one at a time from the elevating mechanism and includes a housing coupled to the isolation valve. A robot arm structure, located in the housing, includes a rotatable plate having extensible and retractable arms with a pallet or spatula at the distal end. With the plate and arms rotated into alignment with the access port and the isolation valve open, the arms are extended to move the pallet into position below a substrate. The arms are raised to lift the substrate on the pallet and out of the cassette, retracted to extract the substrate from the elevating mechanism, rotated to another position and extended once to pass though another isolation valve into a reaction chamber. This handling system relies on the weight of the substrate to hold it in place on the pallet; another prior art structure includes a similar arm arrangement with a vacuum outlet in the pallet for a more positive attachment to the underside of the substrate.
The operation of the above described loading system can be reversed for extracting a processed substrate from the reaction chamber and returning it to the same cassette or to another cassette in a second elevating mechanism for off loading of processed substrates.
While the above describe loading, handling and off loading structures are significantly better than manual operations, they are less than completely satisfactory. One of the prime considerations in modern chemical vapor deposition systems is to minimize contamination or prevent it entirely. Because the vacuum chamber must be opened from time to time for insertion and extraction of cassettes, environmental contamination will occur. The isolation valve prevents contaminants from passing through the vacuum chamber of the elevating mechanism into the housing of the transport system during the time when the vacuum chamber is open to the environment.
A significant problem lies with the robot arm structure. First, such a substrate handling technique cannot possibly place a substrate on a flat continuous surface due to the pallet supporting the bottom surface of the substrate. Therefore, some sort of less than ideal susceptor configuration must be provided in the reaction chamber. Second, damage often results from the mechanical contact of the pallet with the substrate. Contaminants in the form of airborne particles, which may result from the damage or other sources, can settle on the top surface of the substrate, which reduces the yield of the substrates and destroys circuit integrity.
The reaction chambers used in single substrate processing systems may be either a horizontal gas flow system or a vertical gas flow system. The susceptors presently used therein essentially consist of a planar platform and contribute nothing to improve the problems of depletion of the material carried by the reactant gas as it flows past and around the substrate or to improved temperature sensing and control.